"This is a declaration of war against America and all of humanity — it will not stand," Brown said in a statement, according to The Sacramento Bee. "Truth and common sense will triumph over Trump's insanity."

Brown's comments were made on the same day that the Trump administration introduced a proposal, known as the Affordable Clean Energy rule, that would give states more authority when determining regulations and compliance for coal plants.

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The administration's plan would replace the the Obama-era 2015 Clean Power Plan (CPP) regulations that imposed strict federal rules for coal-fired power plants.

Brown has been a strong advocate of CPP since it was introduced three years ago.

California has some of the strictest environmental regulations in the U.S. Brown announced last month that the state's greenhouse gas emissions fell below 1990 levels, meaning the state met its goal to reduce emissions years ahead of schedule.

Brown's criticism of Trump's replacement plan also comes as many Democratic lawmakers voice concerns over the policy.

“If I were grading the Trump Administration’s proposal to replace the Clean Power Plan, I would give it an ‘F,’” Sen. Tom Carper (D-Del.) said in a statement.

The plan to loosen restrictions on emissions from coal-fired power plants would lead to as many as 1,400 premature deaths per year, according to an analysis of the proposal by the Environmental Protection Agency.